1. Field of the Invention
This invention relates to a novel and improved vibration sensing transducer and in particular relates to a geophone having a wide frequency range of operation wherein the damping means includes electrical magnetic damping means to suppress undesired movements of the coil form in a direction substantially parallel to the axis of the permanent magnet.
Vibration sensing transducers, geophones, seismometers, and other similar devices are widely used in a number of applications. One application is in the geophysical field for gathering seismic data used in determining the geophysical characteristics such as used in fossil fuel exploration. Other applications include an intrusion and detecting system wherein ground vibrations generated by personnel or vehicular activity can be sensed and detected.
2. Disclosure of the Prior Art
It is known in the prior art to provide mechanical and electrical damping means for controlling the damping of movable components in a geophone. A geophone senses vibrations by means of transporting a coil of electrical conductors relative to a fixed permanent magnet in response to sensed vibrations. The coil of electrical conductors transverses the magnetic flux lines of a permanent magnet and generates an electrical signal which is responsive in frequency and magnitude to that of the sensed vibration. In such geophones, the coil of electrical conductors is supported by a movable member generally referred to as a coil form. In assignee's U.S. Pat. No. 3,577,184, a coil form which is adapted to rotate within the geophone is disclosed. The coil form disclosed in U.S. Pat. No. 3,577,184 is adapted to support two identical axially aligned coils of electric conductors. The magnitude of the electrical signal generated by the geophone is enhanced by electrically connecting the coils in series aiding.
In assignee's French Pat. No. 2,108,492, a geophone having a coil form adapted for supporting a single coil of electrical conductors is disclosed. The coil form disclosed in French Pat. No. 2,108,492 has a paid of aligned slots around the periphery of one edge of the coil form, which slots cooperate with a pair of pins mounted in the cover of the geophone housing to prevent rotation of the coil form during the sensing of vibrations. In this device, the pins function to prevent rotation of the coil form to avoid exceeding the elastic limit of the springs which, if this occurred, would result in misoperation of the geophone.
The frequency range in which geophones or other vibration sensing transducers are utilized are generally in the order of about one Hz or higher. The frequency range is controllable and is dependent upon a number of factors which include, without limitation, the spring constant (established by the length and width of spring arms, thickness of the spring material, the amount of pre-forming or pre-stressing of the spring arms) and the relationship of the moving mass to the spring constant as defined by the equation w = .sqroot.(K/M). As the resonant frequency of the vibration sensing transducers increases, there is a significant drop in the open circuit damping defined as b.sub.o, which may also be expressed as fraction of critical damping defined as C.sub.c. For purposes hereof, critical damping B is defined by the formula: EQU C.sub.c = 2 .sqroot.KM,
wherein
wherein: K = spring constant, and
M = mass of the coil form.
In the known prior art devices, increasing the mass of the spring mass system generally has the effect of decreasing the damping characteristics of the system. In addition, as the resonant frequency of the system is increased, the decrease in damping is further accentuated. Thus, the prior art devices, when operated at higher frequencies, result in damping characteristics which are substantially lower than desired.